Transport officials said tonight that it could be Monday morning before parts of the fire-damaged M1 can be reopened, creating an ongoing traffic nightmare for motorists.

One northbound lane was reopened at 6.40pm today, but the Highways Agency said it was still investigating the impact of the blaze which tore through a scrapyard beneath the motorway early on Friday.

The fire forced the closure of junctions one to four and left many drivers planning a change of route as they tried to leave London for an extended Easter holiday or football fans heading for this weekend's FA Cup semi-finals at Wembley Stadium.

Around 40 firefighters tackled the blaze which broke out between the M25 and North Circular Road in north London.

A Highways Agency spokesman said urgent repairs are required before the motorway can be fully reopened.

"The viaduct does need to be reinforced before it can carry the weight of traffic and we are putting in supports so we can reopen the road as soon as possible.

"At the same time we are continuing our investigations to determine what remedial work needs to be carried out."

He added: "We expect it to be open by 5am on Monday."

A number of gas cylinders were involved in the blaze and a hazard zone was set up in case they exploded in the heat, London Fire Brigade said.

About 50 people living in properties near the scrapyard were temporarily evacuated as a precaution while the cylinders, some of which were acetylene, were cooled and made safe. Train lines close to the scene were also shut down.

Highways Agency engineers were given access to the site of the fire underneath the motorway and continue to assess the extent of the damage.

According to AA Roadwatch the motorway closure led to congestion on the A1, A40 and A406 as motorists took alternative routes, but the Highways Agency said there were no excessive delays in the vicinity of the shut down.